'DynFlag'-free version of 'mkParserFlags'

Summary:
This is a fixed version of the reverted d2fbc33c
and 5aa29231.
Obtaining a `DynFlags` is difficult, making using the lexer/parser
for pure parsing/lexing unreasonably difficult, even with `mkPStatePure`.
This is despite the fact that we only really need
* language extension flags
* warning flags
* a handful of boolean options
The new `mkParserFlags'` function makes is easier to directly construct a
`ParserFlags`. Furthermore, since `pExtsBitmap` is just a footgun, I've gone
ahead and made `ParserFlags` an abstract type.
Also, we now export `ExtBits` and `getBit` instead of defining/exporting a
bunch of boilerplate functions that test for a particular 'ExtBits'.
In the process, I also
* cleaned up an unneeded special case for `ITstatic`
* made `UsePosPrags` another variant of `ExtBits`
* made the logic in `reservedSymsFM` match that of `reservedWordsFM`
Test Plan: make test
Reviewers: bgamari, alanz, tdammers
Subscribers: sjakobi, tdammers, rwbarton, mpickering, carter
GHC Trac Issues: #11301
Differential Revision: https://phabricator.haskell.org/D5405